1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of efficient toner-concentration control.
2. Discussion of the Background
An image forming apparatus that employs an electrophotographic method has been developed rapidly. Such an apparatus includes a printer, a copier, a facsimile machine, and a multi-function system, for example. Recently, there is increasing demand that such image forming apparatus has high reliability to provide high quality images.
Such image forming apparatus commonly employs the following image concentration control method to maintain an excellent image quality: A gradation pattern to detect toner concentration is formed on an image carrier such as photoreceptor. The gradation pattern is comprised of a plurality of toner patches. The toner patches are formed under different image forming conditions (development potential) to have different toner adhesive amounts. The toner adhesive amount (toner concentration) of each toner patch is calculated using a predetermined adhesive-amount-calculation algorithm with a detection value.
Based on a relation between the toner adhesive amount (toner concentration) and the image forming condition (development potential), development value γ and development-start voltage Vk are obtained. The development value γ is a slope angle and the development-start voltage Vk is an intercept, where a horizontal axis represents the development potential and a vertical axis represents the toner adhesive amount. A LD (laser diode) power, a charging bias, a and development bias are adjusted so that the development potential provides an image forming condition to have an appropriate toner adhesive amount based on the development value γ.
The toner patch is detected by a light sensor that is an optical detection means. The light sensor generally includes an emitting device, such as an LED (Light Emitting Device), and a receiving device, such as photo transistor. For this light sensor, a regular-reflection type light sensor is generally used. In the regular-reflection type light sensor, an output value of the receiving device is high since a regular-reflection light amount is large when a receiving surface of the receiving device is flat. When the receiving surface becomes rougher, the regular-reflection light amount decreases. As a result, an output value of the receiving device is lowered. Specifically, when the toner adhesive amount is small, a large amount of the reflection light is reflected at a flat surface of the image carrier, and the output value of the receiving device increases.
On the other hand, when toner adhesive amount increases, the reflection light decreases because the surface to be detected becomes rougher due to accumulation of toner particles and the output value of the receiving device declines. Thus, there is an inverse relation between the output value from the receiving device and the toner adhesive amount. Accordingly, the toner concentration is known from the output value of the receiving device.
However, the regular-reflection type light sensor may not detect the toner adhesive amount of the toner patch accurately at a high concentration portion. This is because there is little difference between a slight rough state in which toner particles almost cover the surface of the image carrier, and a heavy rough state in which more toner particles adhere and accumulate to form layers.
The light sensor may include two receiving devices. One receiving device receives a regular-reflection light, and another receiving device receives a diffuse-reflection light. Output values of the emitting and receiving devices may change due to temperature dependence and aging of the emitting and receiving devices. Further, the output value of the receiving device is changed due to degradation of the image carrier. Therefore, it is not possible to perform toner concentration detection (toner adhesive amount detection) accurately when the toner adhesive amount is obtained solely from the detection value of the receiving device without adjustment (correction).
The following correction control is generally performed such that the toner concentration (the toner adhesive amount) of each toner patch is obtained from the output value of the receiving device that receives diffuse-reflection light (diffuse-reflection light-sensitive device). A sensitivity correction coefficient α is calculated from the output values of the regular-reflection receiving device and the diffuse-reflection receiving device. Using the sensitivity correction coefficient α, the output value of the regular-reflection receiving device is broken into regular-reflection light element and diffuse-reflection light element. A ratio between an output value at a detection of the surface of the image carrier (output value of background portion) and a regular-reflection light element is determined. Then, the regular-reflection light element is transferred to normalization values β[n] that are values from 0 to 1.
Using a value obtained by multiplying output value of the diffuse-reflection light-sensitive device by the normalization value, diffuse-reflection element from the surface of the image carrier is removed from the output value of the diffuse-reflection light-sensitive device so as to extract the diffuse-reflection element from the toner.
Using the normalization values β[n] and the diffuse-reflection element, the sensitivity correction coefficient n that corrects the output value of the diffuse-reflection light-sensitive device is calculated. The output value of the diffuse-reflection light-sensitive device is corrected by multiplying the diffuse-reflection element from the toner, i.e., an extraction of the output value of the diffuse-reflection light-sensitive device, by the sensitivity correction coefficient η. The toner adhesive amount is then uniquely determined by the output value of the diffuse-reflection light-sensitive device corrected with the sensitivity correction coefficient η.
The output value of the receiving devices are corrected by the sensitivity correction coefficients α and η even when the output values of the receiving devices are changed due to temperature change and aging of the receiving devices. The relation between the output value of the receiving device and the toner adhesive amount is corrected to have a unique relation. Consequently, the light sensor can maintain high-performance detection of the toner adhesive amount accurately by overcoming aging of the receiving devices.
The above-described correction control for the light sensor is achieved by forming a gradation pattern having a plurality of toner patches, i.e., 10 through 16 toner patches, with different adhesive amounts for each color. FIG. 19 is an example of the gradation pattern TK(k), TK(m), TK(c) and TK(y), for each color formed on the intermediate transfer belt that is an image carrier. For this reason, a total length of the gradation pattern including each color pattern increases as shown in FIG. 19. Accordingly, a period from a time the light sensors 310K, 310M, 310C and 310Y start detection to a time the light sensors 310K, 310M, 310C and 310Y end the detection increases. As a result, a correction time to correct the light sensors 310K, 310M, 310C and 310Y increases and a down time of a system increases.
Focusing on the correction time, as shown in FIG. 20, the light sensors 310K, 310M, 310C and 310Y are provided to detect gradation patterns TK(k), TK(m), TK(c), TK(y) of colors K, M, C and Y, respectively. The total length L of the gradation pattern decreases in comparison to the pattern shown in FIG. 19 such that the down time of the system is reduced. However, the cost of the system increases because more light sensors are necessary for each color.
It has been observed that the total length of the toner patches can be shortened, the down time of the system reduced, and toner consumption decreased if a number of toner patches for each color is reduced. Consequently, the inventors have focused their investigations on a light sensor dedicated to detect infrared rays and near-infrared rays, and discovered how to reduce the number of toner patches necessary for correction control.
In conventional systems, toner adhesive amount target values are determined to be equal for each toner patch of the gradation pattern with respect to the colors Y, C, and M. Specifically, three toner patches having equal toner adhesive amounts of different colors are formed as shown in FIG. 21. If the light sensor that detects infrared rays and near-infrared rays is used, three identical detection values are obtained because the reflection rates of the infrared rays and near-infrared rays do not depend on the toner color.
Light sensor correction control to correct the sensitivity of the light sensor using detection results for the gradation pattern is a control to calculate the above-described sensitivity correction coefficients α and η.
The sensitivity correction coefficient α is the minimum value of α ratio between an output value of the regular-reflection receiving device (ΔVsp_reg) and an output value of the diffuse-reflection receiving device (ΔVsp_dif). Specifically, a process to calculate the sensitivity correction coefficient α involves finding the minimum value from the ratios of output value of regular-reflection receiving device (ΔVsp_reg) and output value of diffuse-reflection receiving device (ΔVsp_dif).
Using the light sensor that detects the infrared rays and near-infrared rays, the above-described ratio is changed by differences in toner concentration of the toner patches. However, equal values may be obtained independently for the colors if the toner concentration is equal. Therefore, when a conventional gradation pattern having equal gradation for each color is detected and the above-described ratios are determined using the light sensor that detects the infrared rays and near-infrared rays, three output values are identical.
The sensitivity correction coefficient α is determined by the minimum value of (ΔVsp_reg [n]/ΔVsp_dif [n]). Consequently, calculation accuracy does not improve even with a plurality of data points at equal positions. Therefore, one of the colors Y, M, and C may be enough to calculate the sensitivity correction coefficient α with respect to toner patches P8, P9 and P10 at the high concentrate portion of the gradation pattern.
To obtain the sensitivity correction coefficient η, the data points detected from the toner patch are generally plotted in a graph. In the graph, the horizontal axis represents a normalization value from regular reflection element of output value of the regular-reflection-light receiving device and the vertical axis represents the diffuse-reflection element of output value of the diffuse-reflection-light receiving device.
The sensitivity correction coefficient η is obtained from the plotted line of the toner patches P2, P3 and P4 at low concentration portion. Using the light sensor that detects the infrared rays and near-infrared rays, it is found that the data points are plotted at almost equal positions when the conventional gradation pattern having equal concentration for each color is detected.
Generally, if the data points are uniformly distributed, a plotted line can be recognized correctly, and calculation accuracy of the sensitivity correction coefficient η improves. However, the calculation accuracy does not improve if a plurality of data points are at an equal position. Consequently, with respect to toner patch P2, P3 and P4 at low concentrate portion of the gradation pattern, one of the colors Y, M, and C may be enough to calculate the sensitivity correction coefficient η.
Based on the foregoing discussion, it is possible to calculate the sensitivity correction coefficients α and η if the gradation pattern is formed by one of the colors Y, M, and C for the toner patches at low concentrate portion P2, P3 and P4, and for toner patches at high concentrate portion P8, P9 and P10 using the light sensor that detects the infrared rays and near-infrared rays.
Thus, the number of toner patches can be reduced and the toner consumption can be reduced. Moreover, detection time to detect the toner patches is shortened and it is possible to perform the correction control on the light sensor in a short time, thus reducing down time of the system.
However, one of color toners, i.e., Y, M, and C color toners, may be consumed more quickly than the other color toners, in which case the times to replace the color toner bottles may vary between colors, necessitating replacing the toner bottles frequently.